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1、*P.1 Unit 11Unit 11 Data Compression Chapter Three digital television; streaming media(流媒体 )on the internet; satellite and cable radio; and increasingly in terrestrial(地域性的)radio broadcasts. Lossy compression typically achieves far greater compression than lossless(无损的 ) compression (data of 5 perce
2、nt to 20 percent of the original stream, rather than 50 percent to 60 percent), by discarding(丢弃 ) less-critical data. The innovation of lossy audio compression was to use psychoacoustics(心理声学 ) to recognize that not all data in an audio stream can be perceived by the human auditory(听觉的)system. Most
3、 lossy compression reduces perceptual(知觉的)redundancy(冗余)by first identifying sounds which are considered perceptually irrelevant(不相 关的), that is, sounds that are very hard to hear. Typical examples include high frequencies, or sounds that occur at the same time as other louder sounds. Those sounds a
4、re coded with decreased accuracy or not coded at all. & Part4 Extensive Reading *P.12 While removing or reducing these unhearable sounds may account for a small percentage of bits saved in lossy compression, the real savings comes from a complementary phenomenon - noise shaping. Reducing the number
5、of bits used to code a signal increases the amount of noise in that signal. In psychoacoustics based lossy compression, the real key is to hide the noise generated by the bit savings in areas of the audio stream that cannot be perceived. This is done by, for instance, using very small numbers of bit
6、s to code the high frequencies of most signals - not because the signal has little high frequency information (though this is also often true as well), but rather because the human ear can only perceive very loud signals in this region, so that softer (noise) sounds hidden there simply arent heard.
7、*P.13 If reducing perceptual redundancy does not achieve sufficient compression for a particular application, it may require further lossy compression. Depending on the audio source, this still may not produce perceptible differences. Speech for example can be compressed far more than music. Most lo
8、ssy compression schemes allow compression parameters to be adjusted to achieve a target rate of data, usually expressed as a bit rate. Again, the data reduction will be guided by some model of how important the sound is as perceived by the human ear, with the goal of efficiency and optimized(使最优化) q
9、uality for the target data rate. (There are many different models used for this perceptual analysis, some better suited to different types of audio than others.) Hence, depending on the bandwidth and storage requirements, the use of lossy compression may result in a perceived reduction of the audio
10、quality that ranges from none to severe, but generally an obviously audible reduction in quality is unacceptable to listeners. *P.14 Because data are removed during lossy compression and cannot be recovered by decompression, some people may not prefer lossy compression for archival storage. Hence, a
11、s noted, even those who use lossy compression (for portable audio applications, for example) may wish to keep a losslessly compressed archive for other applications. In addition, the technology of compression continues to advance, and achieving a state-of-the-art(最先进的)lossy compression would require
12、 one to begin again with the lossless, original audio data and compress with the new lossy codec(编解码器). The nature of lossy compression for both audio and images results in increasing degradation of quality if data are decompressed, then recompressed using lossy compression. End *P.15 Part5 Notes to
13、 Passage 1)The innovation of lossy audio compression was to use psychoacoustics to recognize that not all data in an audio stream can be perceived by the human auditory system.有损压 缩音频的创新在于运用心理声学来认识这一事实，即：并非所有音 频流中的数据都能被人类的听觉系统所察觉。 注释：此句是主系表句型。to use psychoacoustics是表语；to recognize是表语里的目的状语。that not
14、all data in an audio stream can be perceived by the human auditory system是宾语 从句。 2)Most lossy compression reduces perceptual redundancy by first identifying sounds which are considered perceptually irrelevant, that is, sounds that are very hard to hear. 大部分的有 损压缩首先是通过识别出那些我们认为无关的声音，也就是那些很 难听的声音，来减少知
15、觉的冗余。 注释：介词短语by first identifying sounds用作方式状语；which are considered perceptually irrelevant和that are very hard to hear 都是定语从句，分别修饰前后两个sounds。 *P.16 End 3)While removing or reducing these unhearable sounds may account for a small percentage of bits saved in lossy compression, the real savings comes f
16、rom a complementary phenomenon - noise shaping.去掉或减少这些“没法听”的声音固 然能够在有损压缩过程中节省一小部分位数，但真正有助于有损压 缩节省大量位数的是一个互补现象噪音抑制。 注释：注意While引导的是状语从句，意为“虽然、固然、尽管”表示 让步；所谓noice shaping“噪音抑制”或“噪音修整”，是一种处理 音频噪音的专门技术 4)Most lossy compression schemes allow compression parameters to be adjusted to achieve a target rate of data, usually expressed as a bit rate. 大多数的有损压缩方案都允许通 过调整压缩参数来达到某个目标数据比率，通常用比特率来表示。 注释：usually expressed as a bit rate是过去分词做定语，相当于 which is usually expressed as a bit rate。 *P.17 1.List
